Builders often talk about the R-value of their walls. But if a builder claims to have an R-20 wall, what does that mean?
Building codes commonly include a table listing the minimum prescriptive R-values for walls and ceilings in different climate zones. For example, Table R402.1.1 in the 2012 International Energy Conservation Code (IECC) informs builders that the minimum prescriptive R-value for walls in Climate Zones 3, 4, and 5 is “20 or 13+5.”
This type of table raises many questions. For example, if a builder chooses to comply with the R-20 option, how is R-20 calculated? The code provides some guidance on the issue, but not much. According to a footnote at the bottom of the table, the “first value is cavity insulation, second is continuous insulation or insulated siding, so ‘13+5’ means R-13 cavity insulation plus R-5 continuous insulation or insulated siding.”
Complying with the code usually means installing insulation between the studs
The code language governing the prescriptive R-value requirements has changed in recent years. If you want to know the specific language that is enforced in your jurisdiction, you’ll have to consult your local code book. For example, the prescriptive requirements in the 2009 International Residential Code (IRC) note (in section N1102.1.1), “Computed R-values shall not include an R-value for other building materials or air films.”
A footnote to — equivalent in most respects to the prescriptive table (Table R402.1.1) in the 2012 IECC — notes, “R-19 batts compressed in to nominal 2Ã—6 framing cavity such that the R-value is reduced by R-1 or more shall be marked with the compressed batt R-value in addition to the full thickness R-value.” This footnote is confusing. Who should do the marking? Does this instruction mean that the builder has to mark the insulation in every stud bay? How do you mark an unfaced batt — with spray paint? Or is this footnote directed at insulation manufacturers?
R-19 batts provide about R-18, if you ignore thermal bridging
The footnote about R-19 batts traces its roots to a time when codes required walls in some climate zones to be insulated to R-19. To comply with this requirement, builders usually used 6-inch-thick R-19 batts (originally developed for use on attic floors). Since these batts were labeled “R-19,” many inspectors accepted them — even though, when compressed to 5 1/2 inches, the batts only provided R-18. (For the time being, we’re not even going to address the fact that thermal bridging through the studs reduces the thermal performance of this type of wall to about R-13). This is the issue that code writers were attempting to address with their confusing footnote about “marking” the insulation.
In a later edition of the code (the 2012 IECC), this footnote was changed to read, “When insulation is installed in a cavity which is less than the label or design thickness of the insulation, the installed R-value of the insulation shall not be less than the R-value specified in the table.” In other words, a builder who installs an R-19 batt in a 5 1/2-inch cavity is supposed to know that the (slightly compressed) batt is actually an R-18 batt.
It would be nice if code writers knew how to write English
These examples point to a basic problem: The code is so poorly written that our chief guide through the murky thicket is oral tradition.
“I’m embarrassed by how poorly the code language is chosen,” Joe Lstiburek told me recently. Lstiburek is a principal at Building Science Corporation in Massachusetts who has successfully shepherded several code-change proposals through the Byzantine code-approval process. “It’s horrible, but it is the best we can do. To paraphrase Winston Churchill, the energy code is the worst form of code writing there is except for all of the others. We keep hearing from people who say, ‘We want the code to be simple.’ It isn’t supposed to be a physics or engineering document. Builders just want to know to put in an R-20 batt. They don’t care about thermal bridging. Everybody know what ‘R-20’ means. The only problem is that technological innovation has made this approach obsolete. So how do you fix the way the code is written? It’s almost impossible to fix, because it takes forever, and nobody has an interest in changing the code. The way you get codes changed is that somebody pays you a lot of money to work on getting some aspect of the code changed. Manufacturers pay lobbyists to do that. But nobody has a financial interest in improving bad language in the code. And if you change any aspect of the language, there is almost always somebody who will be offended.”
Not all walls with R-20 insulation are equal
There are several problems with the traditional method used by building inspectors to define an R-20 wall. The most obvious problem is that the R-value of a wood-framed wall is always less that the R-value of the batts installed between the studs.
Another problem is that some walls insulated with R-20 batts perform better than other walls insulated with R-20 batts. For example:
- Walls with 24-inch-on-center studs have a lower framing factor (and therefore a higher whole-wall R-value) than walls with 16-inch-on-center studs.
- Walls with single top plates have a lower framing factor (and therefore a higher whole-wall R-value) than walls with double top plates.
- Walls with insulated headers have a higher whole-wall R-value than walls with uninsulated headers.
- Walls that are well sealed perform better than walls that have a lot of air leaks.
Using R-19 batts to comply with the R-20 requirement
To address some of these issues, APA — The Engineered Wood Association (a group formerly known as the American Plywood Association) is now arguing that builders who reduce the number of studs in their walls, or who install insulated headers over windows and doors, should be able to use R-19 insulation and still get credit for an “R-20” wall.
Apparently, one of the reasons that APA is taking this position is that the organization is worried about the increasing use of foam sheathing. If the use of foam sheathing causes a reduction in the demand for OSB and plywood, the manufacturers that APA represents will lose business.
The logic behind the APA’s suggested approach is explained in a new APA document, Since the document was reviewed by the International Code Council (ICC) before publication, it includes a stamp of approval from the ICC. The document states, “The assemblies in this publication are deemed to be equivalent to the frame wall assemblies of the prescriptive insulation requirements specified in Table R402.1.1 of the 2009, 2012, and 2013 IECC for those climate zones that require R-20 or R-13+5 insulation.”
However, there’s at least one problem with this explanation: Unfortunately, there are no “frame wall assemblies” listed or described in the prescriptive insulation requirements specified in Table R402.1.1 — just a very vague table with a few obfuscatory footnotes.
The U-factor alternative path
The recommendations made in the APA document are apparently based on one of the four compliance paths offered by the IECC — namely, the U-factor alternative path (found in section 402.1.3 of the 2009 IECC and section N1102.1.2 of the 2009 IRC). The calculations are based a few assumptions; these include the assumption that the typical framed wall has a framing factor of 25%, and that any wall with a lower framing factor can use insulation with a lower R-value than shown in the prescriptive table and can still end up with an equivalent whole-wall R-value.
Another key to the APA’s calculation is its assertion that “the equivalent U-factor for R-20 or R-13+5 wall assemblies will be 0.060 in the 2013 code.” A U-factor of U-0.06 is equivalent to a whole-wall R-value of R-16.6. The APA argues that wall assemblies with a whole-wall R-value of R-16.6 are equivalent to walls that meet the R-20 prescriptive requirement.
So, what if a builder wants to use R-19 batts, which are cheaper than R-20 batts? Here’s what you do, according to APA: you use advanced framing methods, and give yourself credit for having a lower framing factor than other walls. You also give yourself credit for air films, for vinyl siding, for 7/16-inch OSB, and for drywall. (We’ll leave aside, for the time being, the fact that the 2009 IECC specifies in section N1102.1.1 that “Computed R-values shall not include an R-value for other building materials or air films.” Presumably, that limitation applies only to builders following the prescriptive path, not to builders following the U-factor alternative path. But wouldn’t it be nice if the code were written clearly enough that builders didn’t have to guess what the code means?)
Here’s how it adds up: Vinyl siding is R-0.59; OSB is R-0.62; fiberglass batts are R-18; two air films and 1/2-inch drywall are R-1.38. Then you adjust your calculation to take into account the framing lumber. You assume a framing factor of 20% for advanced framing, and assume that the framing has an R-value of R-6.88. Using the two-dimensional assembly U-factor calculation method outlined in ASHRAE Fundamentals, the wall assembly is determined to have a U-factor of 0.06 and a whole-wall R-value of 16.67.
R-19 batts are significantly cheaper than R-20 batts
What’s the point of this exercise? For production builders worried about every penny, this approach allows you to downgrade from R-20 batts to less expensive R-19 batts. And for the APA, it provides an inexpensive option for some builders who may be tempted to install rigid foam sheathing — a threat to the market for OSB and plywood. As the APA points out, “All of the assemblies shown in this guide include the use of continuous minimum 7/16 Performance Category wood structural panel wall sheathing.”
The APA guide includes several more suggested wall assemblies that use less insulation than required by the prescriptive table in the code, and yet still comply with the IECC.
Code compliance options
If the complexity of the above arguments leaves you shaking your head, it’s worth pointing out that there are several more ways to comply with the energy code.
Most versions of provide four compliance paths:
- The prescriptive path, which requires builders to follow the simple table that lists minimum R-values for walls, ceilings, and floors in different climate zones;
- The U-factor alternative — the approach used by APA to argue in favor of certain wall assemblies that use R-19 batts;
- The Total UA alternative; and
- The simulated performance compliance path.
The “Total UA” alternative path
The “Total UA” alternative compliance path is described in section R402.1.4 of the 2012 IECC:
“R402.1.4 Total UA alternative. If the total building thermal envelope UA (sum of U-factor times assembly area) is less than or equal to the total UA resulting from using the U-factors in Table R402.1.3 (multiplied by the same assembly area as in the proposed building), the building shall be considered in compliance with Table R402.1.1. The UA calculation shall be done using a method consistent with the ASHRAE Handbook of Fundamentals and shall include the thermal bridging effects of framing materials. The SHGC requirements shall be met in addition to UA compliance.”
Builders who choose the Total UA Alternative path can choose insulation thickness and window U-factors that deviate from those in the prescriptive table, as long as the total building thermal envelope UA (the sum of each assembly U-factor times each relevant area) isn’t more than the total UA resulting from using the U-factors in the prescriptive table. If builders want to follow this path, they usually hire an energy consultant to perform the necessary calculations.
Simulated performance path
The simulated performance path uses computer software to calculate an energy budget for a proposed home design. Confusingly, the code calls this compliance path “performance-based compliance,” even though it is based not on performance, but on computer modeling.
This path is described in section 405.3 of the 2009 IECC: “405.3 Performance-based compliance. Compliance based on simulated energy performance requires that a proposed residence (proposed design) be shown to have an annual energy cost that is less than or equal to the annual energy cost of the standard reference design. Energy prices shall be taken from a source approved by the code official, such as the Department of Energy, Energy Information Administration’s State Energy Price and Expenditure Report. Code officials shall be permitted to require time-of-use pricing in energy cost calculations.”
Builders following this path must use energy modeling software to show that a proposed house design has an annual energy budget (in dollars) less than or equal to that of a similar house (known as the “standard reference design”) that complies with the code’s prescriptive requirements.
What does the code say about slab insulation?
Insulation requirements for slab-on-grade floors can be found in section R402.2.9 of the 2012 IECC. (Similar provisions in the 2009 IRC can be found in section N1102.2.8.) According to that section, “Slab-on-grade floors with a floor surface less than 12 inches below grade” are required to be insulated, and the insulation should comply with the requirements of the prescriptive table (that is, Table R402.1.1 in the IECC, or Table N1102.1 in the IRC).
The code is silent concerning whether above-grade slabs or slabs that are more than 12 inches below grade need to be insulated. The vast majority of slab-on-grade floors are actually above grade rather than below grade; the code implies that above-grade slabs don’t need to be insulated.
That interpretation doesn’t correspond with oral tradition, but that is what the code says.
Most of the compliance methods described above are flawed. Some — especially the prescriptive method laid out in the table that tells builders to install insulation between studs — are deeply flawed. Others — including the simulated performance path — are only a little bit flawed.
Joe Lsiburek’s pessimistic conclusion — that badly written building codes are “almost impossible to fix” — is probably correct. For years to come, we’re going to be stuck with the International Residential Code and the International Energy Conservation Code, flaws and all. So we might as well learn as much as we can about our different compliance options.
Martin Holladay’s previous blog: “EMFs and Human Health.”